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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

224

ORDINARY BRASSES

ORDINARY   BRASSES

Ordinary brasses are alloys of copper and zinc in various proportions
always containing as impurities small quantities of lead, iron, tin and some-
times nickel, arsenic, sulphur, phosphorus and bismuth. Their analysis
comprises the determination, by electrolytic or gravimetric methods, of
the constituent elements and impurities.

A. Electrolytic Methods

In a covered beaker, tall and narrow, I gram of the alloy in turnings or
filings is dissolved at a gentle heat in 15 c.c. of nitric acid (D 1-2).

1.  Determination of the Tin.—The solution is diluted with 30-40 c.c.
of water and if it appears turbid owing to the presence of metastannic acid,
it is evaporated to dryness, taken up in a few drops of nitric acid and a
little water, heated for some time and filtered, the insoluble residue being
collected on a small, tight filter and the nitrate in a 350 c.c. electrolytic
beaker.   The filter is washed, first with water faintly acidified with nitric
acid and then with water alone, dried, ignited in a porcelain crucible and
weighed *:  Sn02 X 0-7881 = Sn.

2.  Determination of the Copper and Lead.—The liquid freed from
the metastannic acid is treated with 15 c.c. of nitric acid of D 1-2 (in absence
of tin, it is not necessary to add fresh acid, since the evaporation of the
solution to diyness is omitted), made up to about 200 c.c., allowed to cool
and electrolysed to determine simultaneously the copper and the lead 2 :
Winkler cathode and matte gauze cylinder anode ; ND100 = 0-3  amp. ;
voltage, 1-8-2 ;   duration, 10-15 hours.    After half an hour or an hour,
when the greater part of the lead has been deposited as peroxide on the
anode, 20 c.c. of 10% sulphuric acid are added to the electrolyte, which is
carefully stirred by means of a glass rod.

When the deposition of the copper and lead is complete, the electrolytic
beaker is replaced, rapidly and without interruption of the current, by a
small, tall and narrow beaker containing water acidified with sulphuric
acid, this being replaced after some time by another beaker containing dis-
tilled water. Finally the electrodes are detached and washed, the cathode
with water, alcohol and ether, followed by drying at 70°, and the anode
with water alone, followed by drying at 180-200° : Pb02 x 0-866 = Pb.

3.  Determination of the Iron and Zinc.—The liquid from which
the copper and lead have been separated and the wash water from the
first beaker are evaporated together until white fumes of sulphuric acid
are emitted, in order to convert the zinc and iron nitrates into sulphates.
When cold, it is taken up in water acidified with sulphuric acid, a few drops
of hydrogen peroxide added, the liquid heated to convert the basic sulphates

1  For the electrolytic determination of tin, see Ordinary Bronzes.

2  If the alloy contains bismuth, arsenic and manganese, the determination of the
lead cannot be effected simultaneously with that of the copper, since these metals may
be deposited on the anode.    In this case the lead must be determined by the method
indicated for the analysis of delta metal (see later : Complex Brasses).ytic copper